In reply to Pete_Frost: Just to add to what others have said above, whilst the practice of using a single rope and a tag line is well documented I wouldn't (in my experience at least) say that is it 'common practice'. The advent of skinny 8mm half ropes, and 7.5mm twins means that it is usually easier, more efficient and safer to just take two ropes (or to just take one and abseil lots or not retreat).
It does work, but pulling the line through adds a lot of friction (it's hard work), and there's more chance of the knot jamming. Any cord thinner than 5 or 6mm is difficult to pull on hard, and also tangles badly (as well as being a bit too thin to abseil on itself if required). Using two 'proper' 7.5mm twins adds little in terms of weight, but allows easier abseils and adds redundancy which is useful if ascending/descending technical or mixed ground - I have killed three icelines on alpine routes (and been glad I had redundancy in the system), but have never felt the need to save 200g for the added hassle it would cause.
Having said that, the system would work well in some circumstances, such as a solo descent of non-technical and moderate angled ice where weight saving was at a premium and jamming of, or damage to the rope unlikely.